1. Field of the Invention
This invention relates to valve gates used in injection molding of thermoplastic material, and more particularly relates to a position detector to indicate the fully opened position of such a valve gate.
2. Description of the Prior Art
In the injection molding of plastic parts, such as audio or video cassette casings, which are manufactured in great quantities, a plurality of injection mold cavities are used in the molding process. Typically the flow of molten plastic material to the injection mold cavities is controlled by valve gates which have valve pins reciprocating between a closed position precluding the flow of molten plastic material, and an open position permitting molten plastic material to flow through the valve gates to respective mold cavities. Where the flow of material to a multiplicity of injection mold cavities is controlled by a number of valve gates, it is necessary that all of the valve pins in all of the valve gates be in a fully opened position before molten plastic flows. This is necessary to avoid underfilling or overfilling any injection mold cavity.
Heretofore, problems have developed in such situations where one or more valve gates are not in a fully opened position at the time molten plastic material is introduced in the injection molding process. When not all valve gates are in a fully opened position, underfilling of selected injection mold cavities and overfilling of adjacent mold cavities results so that some parts being molded are defective resulting in undesirable parts which must be scrapped or even in damage to the mold cavities themselves.